Soda ash or sodium carbonate is an inorganic salt made from the mineral trona. Soda ash is one of the largest volume alkali commodities made in the United States. Soda ash finds major use in the glass-making industry and for the production of baking soda, detergents and paper products.
Large deposits of the mineral trona in southwestern Wyoming near the Green River Basin have been mechanically mined since the late 1940's. In 2007, trona-based sodium carbonate from Wyoming comprised about 90% of the total U.S. soda ash production. Trona ore is a mineral that contains about 70-99% sodium sesquicarbonate dihydrate (Na2CO3.NaHCO3.2H2O). Trona ore contains insoluble matter in the form of shale. The shale contains various constituents such as organic kerogeneous matter (e.g., 0.1-1% as carbon) and dolomitic and silica bearing materials (e.g., about 5-15%), such as dolomite, quartz, feldspar, clay.
The crude trona is normally purified to remove or reduce impurities, primarily shale and other water insoluble materials, before its valuable sodium content can be sold commercially as: soda ash (Na2CO3), sodium bicarbonate (NaHCO3), caustic soda (NaOH), sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O), sodium sulfite (Na2SO3), a sodium phosphate (Na5P3O10), or other sodium-containing chemicals.
To recover these valuable alkali products, the ‘monohydrate’ commercial process is frequently used to produce soda ash from trona. In the production of soda ash, crushed trona ore is calcined (e.g., heated) to decompose the sodium sesquicarbonate to sodium carbonate.2Na2CO3.NaHCO3.2H2O→3Na2CO3+5H2O(g)+CO2(g)
The calcination drives off water of crystallization and forms crude soda ash. During calcination, a part of the water insoluble silicate bearing material contained in the ore is converted to soluble silicates. The calcined ore is dissolved in water or dilute sodium carbonate liquor to give a saturated solution of ˜30 wt. % Na2CO3 (depending upon the temperature of the solution) containing water-soluble impurities. The water-soluble impurities may comprise silicates, organics, sodium chloride, and sodium sulfate. The insoluble material is separated from the resulting saturated solution. This clear sodium carbonate-containing solution is fed to an evaporative crystallizer. As this solution is heated, evaporation of water takes place effecting the crystallization of sodium carbonate into sodium carbonate monohydrate crystals (Na2CO3.H2O). The monohydrate crystals are removed from the mother liquor and then dried to convert it to anhydrous soda ash (Na2CO3). The mother liquor is recycled back through a crystallizer circuit for further processing into sodium carbonate monohydrate crystals.
The crystallization step however concentrates impurities in the mother liquor. Indeed, by the effect of water evaporation, the soluble impurities such as organics, silicates, sodium chloride, and sodium sulfate, become concentrated in the crystallizer. If this is allowed to continue, eventually the concentration of the impurities builds to a point where the resulting sodium carbonate product quality may be negatively impacted. Applicants have found for example that the presence of water-soluble silicates in the crystallizer liquor seems to impact the morphology of crystallization and may diminish the yield of salable product. Additionally, the presence of impurities in the crystallizer liquor can cause severe scaling of the surfaces of equipment in which this saturated solution is handled, for examples lines, tanks, pumps, and particularly the crystallizer heat-exchanger which handles the liquor in a recycling loop connected to the crystallizer. For example, an accumulation of sodium chloride and/or sulfate, both impurities originating from crude trona, in the crystallizer liquor can result in the formation of complex salts which may crystallize out of the hydrated product. A scale buildup containing such impurities generally formed on exposed surfaces of the crystallizer heat exchanger requires frequent and expensive high pressure washes.
Therefore, to maintain the concentration of the water-soluble impurities below the crystallization point so as to avoid contamination and deterioration of crystal shape, size, and hardness by the impurities and to prevent the buildup of these impurities in the crystallizer, a portion of the crystallizer liquor must be purged, and is generally called ‘purge liquor’. This can result in a loss of up to about 10% of the soda values. The purge liquor exiting a crystallizer typically includes sodium carbonate and/or sodium bicarbonate, as well as impurities, such as water-soluble organics, sodium chloride, sodium sulfate, and silicates. A purge liquor exiting a monohydrate crystallizer may contain ca. 19-30% sodium carbonate, 0.1-4% sodium bicarbonate, 0.2-1% silicates, up to 2.7% sodium chloride, up to 2.4% sodium sulfate, and 100-1,500 ppm Total Organic Carbon (TOC), but typically contains ca. 23-28% sodium carbonate and 0.2-3% sodium bicarbonate. A purge liquor exiting a bicarbonate crystallizer may contain ca. 5-25% sodium carbonate, 1-15% sodium bicarbonate, up to 1% silicates, up to 2.7% sodium chloride, up to 2.4% sodium sulfate, and 100-1,500 ppm TOC.
In the manufacture of soda ash and/or sodium bicarbonate, a system of storage ponds has been used to accommodate disposal of the plant effluent stream including mine water, crystallizer purge liquor, and other sources of waste water effluents inherent to the process. The effluent stream is transported year-round to at least one pond. The purge liquor exiting the crystallizer is typically stored in one or more tailings (waste) ponds which use up large areas of land. The pond area is generally capable of handling a depth of at least six feet (1.83 meters) high of effluent, and would typically be from 1 to 100 acres (4,047-405,000 m2) in surface area. During the summer, water evaporates from the pond resulting in the crystallization of a sodium salt (e.g., mainly sodium carbonate decahydrate when a major source of the plant effluent stream is a monohydrate crystallizer purge), which is contaminated with varying amounts of impurities, including silicates, sodium chloride, sodium sulfate, and organics, the impurities content in the crystallized sodium salt (especially when it is sodium carbonate decahydrate) generally being smaller than in the pond solution. In the winter, crystals form by cooling crystallization, and are deposited at the bottom of the pond over the previously deposited summer precipitation. The evaporation process also concentrates the amount of sodium chloride and other impurities in the pond solution. The supernate liquid in the continuously filled ponds typically has NaCl levels of greater than 11% in the summer and 14% in the winter. Such pond deposit reduces the total pond volume. The solids depth may vary, but solid depth of about 1.2 m (4 ft) or more have been observed. There is usually a depth of about 0.3-0.6 m (1-2 ft) of liquor on top of the pond deposit. Because the pond deposit generally contains a large portion of sodium carbonate decahydrate, it is generally referred to as a ‘deca’ deposit. However in the manufacture of soda ash and/or sodium bicarbonate, the pond deposit may further contain sodium bicarbonate and/or sodium sesquicarbonate. A ‘deca’ deposit thus in the pond would not only contain sodium carbonate decahydrate but also would contain sodium bicarbonate and/or sodium sesquicarbonate; and the content in sodium carbonate in the ‘deca’ solid would generally be greater than the content in sodium bicarbonate.
In some instances, a series of ponds may be employed, one of them being a crystallization pond in which a deposit is formed, such deposit being lean in impurities. The remaining spent liquor from the crystallization pond (supernatant above the deposit) may be pumped, drained by pipes or by overflow to a mother liquor pond, in which there is also a formation of another deposit, such deposit being laden with impurities.
The quality of the pond deposit can vary greatly within a specific pond and from pond to pond. For the most part, the ‘deca’ deposit with the highest quality and easiest to recover due to its ‘softness’ is located in a section of the crystallization pond where the plant effluent (comprising a sodium carbonate monohydrate purge liquor) is flooded over in the wintertime to form sodium carbonate decahydrate by cooling crystallization. Another section of the crystallization pond may contain “hard” deca deposit, which is generally of lower quality and is more difficult to recover. Because the mother liquor pond is generally fed by a “run off” from the crystallization pond, both the pond liquor and the pond deposit in the mother liquor pond have very high impurities contents, that is to say much higher than the deposit from the crystallization pond. Due to the differences in the impurities level in such deposits, in some instances, the deposit formed in the crystallization pond may be referred to as ‘impurities-lean’ deposit, while the other deposit formed in the mother liquor pond may be referred to as ‘impurities-laden’ deposit.
If the pond deposit is not removed from a pond, it eventually fills the available pond volume until an increase in pond volume must occur, such as by raising existing dikes, expanding the existing pond, or by constructing a new pond. It would be beneficial to recover and use the pond deposit mass from the tailings pond(s), as the removal of this solid mass would free up previously filled volume in the tailings pond(s). Since the recovered pond deposit mass contains valuable sodium carbonate content that would otherwise have to be mined, it would be beneficial to recycle the recovered pond solid to a process for the manufacture of soda ash, sodium bicarbonate, and/or other valuable sodium-containing derivative products (such as sodium sulfite).
However it was observed that while some ‘deca’ deposit (‘impurities-lean’ deposit) may have low content in NaCl and/or Na2SO4 (e.g., lower than 2 wt. %), other pond deposits do have quite an excessive amount of NaCl and/or Na2SO4 (e.g., higher than 2 wt. % or even higher than 4 wt. %) and are not currently being recovered to be repurposed because the sodium chloride and/or sodium sulfate content is too high which would cause operational upsets and may cause product quality issues.
Additionally, Applicants have found that the propensity of fines formation in a soda ash product was even greater when a ‘deca’ pond solid was recycled to a soda ash process such as by dissolving it in an aqueous medium to serve as a feedstock to the sodium carbonate monohydrate crystallizer. It is believed that the soda ash product degradation was due to the higher amount of impurities being carried over from the recovered pond solid. For example, it has been found that while a calcined trona liquor may contain about 70 ppm silica, a recovered ‘deca’ pond solid may contain about 600 ppm silica. When 10% of the crystallizer feedstock is made from dissolved recovered ‘deca’ pond solid with the remainder being the dissolved calcined trona, there is almost a doubling in the ppm silica level in the resulting solution. Consequently, the impact of impurities is even more felt when the recovered ‘deca’ pond solid is recycled to the soda ash plant as a feedstock for crystallization.
Applicants have further observed that propensity of foam formation in the sodium carbonate monohydrate crystallizer was an additional operational issue which was even more prominent when a ‘deca’ solid recovered from a tailings crystallization pond was recycled to the soda ash process to serve as part of a monohydrate crystallizer feed. It is suspected that the greater foam incidence with this recycle was due to water-soluble organics which were carried over from the recycled ‘deca’ solid.
In addition to the soda ash production process, other processes utilizing saturated or near-saturated sodium carbonate-containing solutions as feedstocks to make derivative products may be impacted by water-soluble impurities present in such solution, particularly if a portion of such solution contains a dissolved pond solid such as a recovered solid comprising sodium carbonate decahydrate. Examples of such processes include a sodium sulfite production process which may use a sodium carbonate-containing solution as feedstock to the sulfite reactor, and/or a sodium bicarbonate production process which may use a sodium carbonate-containing solution as feedstock to the bicarbonate reactor. Such process includes forming sodium sulfite or bicarbonate by reaction of a sodium carbonate-containing solution with sulfur dioxide or carbon dioxide gas, respectively. Sodium sulfite crystals are typically formed in a sulfite crystallizer, while the sodium bicarbonate crystals are typically formed in the bicarbonate reactor at the same time as the reaction with CO2 takes place.
Since the sodium carbonate feedstock contains water-soluble impurities, these impurities concentrate and precipitate in the sodium sulfite or bicarbonate process which may negatively impact the final product quality. For example, there are quality specifications limiting water insoluble matter in photo-grade sodium sulfite imposed by ISO 418 Photography—Processing chemicals—Specifications for anhydrous sodium sulfite. The removal of impurities before they can contaminate the final crystalline sodium sulfite product would allow the sodium sulfite process which uses dissolved calcined trona and/or dissolved pond solid (such as comprising sodium carbonate decahydrate) as sodium carbonate feedstock(s) to make a photo-grade sodium sulfite.
It is thus apparent that a need exists for a more effective method for reusing a waste solid in a process to form a final product comprising sodium carbonate, bicarbonate and/or sulfite, but wherein the waste contains water-soluble impurities which may cause a negative impact on product quality and/or on operation of the process. A need exists for obtaining a less-friable crystalline anhydrous sodium carbonate product or a photo-grade crystalline sodium sulfite or a bicarbonate product with a reduced impurity content from an impure feedstock solution which comprises water-soluble impurities (e.g., sodium chloride, sodium sulfate, silicates, organics) originating from the recovered waste solid and/or from calcined trona ore. There is also a need to minimize operating costs by reducing downtime for maintenance of equipment which is exposed to impurities (due to cleaning of scale). There is also a need to minimize loss of soda ash values by recycling a crystalline sodium-containing deposit recovered from a waste pond and carrying impurities, yet without impacting the quality and yield of salable soda ash product or the quality of other derivatives (e.g., sodium sulfite, sodium bicarbonate, sodium sesquicarbonate) which are made from sodium carbonate-containing solutions, particularly those comprising the recycled waste solid. There is also a need to reduce operational issues such as scaling of equipment, foaming in crystallizer(s) created at least in part from the reintroduction of impurities such as sodium chloride, sodium sulfate, silicates and/or organics when recycling a waste solid recovered from a tailings pond and optionally from a secondary crystallizer.